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| United States Patent |
5,323,119
|
|
Powell
, et al.
|
June 21, 1994
|
Amplifier having feed forward cancellation
Abstract
The present invention provides an amplifier arrangement to which feed
forward correction is applied by a comparison loop including comparison
means for comparing amplifier input with amplifier output to provide an
error signal, a cancellation loop including secondary amplifier means for
amplifying the error signal and combining means for combining said
amplified signal with said amplifier output, a pilot generator coupled to
said amplifier input to introduce a pilot tone therein, detector means for
detecting a level of pilot tone in said amplifier output and correction
means for correcting said cancellation loop performance as a function of
said detection wherein said pilot generator is further coupled to a
multiplier receiving said amplifier output, said multiplier producing an
output signal arranged to control a loop parameter to effect said
correction. The distortion performance of a feed forward amplifier is
thereby improved, alternatively an amplifier of inferior distortion
performance, but potentially higher efficiency may be used.
| Inventors:
|
Powell; Jack (Cestas, FR);
Ha; Thomas (Gardena, CA);
Luettgenau; Georg (Madrid, ES)
|
| Assignee:
|
Motorola, Inc. (Schaumburg, IL)
|
| Appl. No.:
|
690927 |
| Filed:
|
July 14, 1991 |
| PCT Filed:
|
November 12, 1990
|
| PCT NO:
|
PCT/EP90/01890
|
| 371 Date:
|
July 14, 1991
|
| 102(e) Date:
|
July 14, 1991
|
| PCT PUB.NO.:
|
WO91/07812 |
| PCT PUB. Date:
|
May 30, 1991 |
Foreign Application Priority Data
| Current U.S. Class: |
330/151 |
| Intern'l Class: |
H03F 001/26 |
| Field of Search: |
330/52,149,151
|
References Cited
U.S. Patent Documents
| 3922617 | Nov., 1975 | Denniston et al. | 330/151.
|
| 4885551 | Dec., 1989 | Myer | 330/151.
|
Primary Examiner: Mullins; James B.
Attorney, Agent or Firm: Barbee; Joe E., Botsch, Sr.; Bradley J.
Claims
We claim:
1. An amplifier arrangement including an amplifier to which feed forward
cancellation is applied, comprising:
a comparison loop including comparison means for comparing an amplifier
input signal with an amplifier output signal to provide an error signal,
a cancellation loop including
(i) secondary amplifier means for amplifying the error signal,
(ii) combining means for combining said amplified error signal with said
amplifier output signal, and
(iii) correction means coupled between said secondary amplifying means and
said combining means for correcting performance of said cancellation loop;
a pilot generator coupled to said amplifier input to introduce a pilot tone
thereat,
detector means for detecting a level of said pilot tone in said amplifier
output signal, said detector means including a first multiplier for
receiving both said amplifier output signal and said pilot tone for
providing a first output signal to said correction means to control a
first loop parameter, said detector means including a second multiplier
for receiving both said amplifier output signal and a phase shifted
version of said pilot tone for providing a second output signal to said
correction means to control a second loop parameter.
2. An amplifier arrangement as claimed in claim 1 and wherein said first
loop parameter is amplitude.
3. An amplifier arrangement as claimed in claim 1 and wherein said second
loop parameter is phase.
4. An amplifier arrangement as claimed in claim 1 and including means for
controlling a parameter of the comparison loop.
5. An amplifier arrangement as claimed in claim 4 and wherein said loop
parameter is phase.
6. An amplifier arrangement as claimed in any one of the preceding claims
and including means for modulating the pilot tone and means for
demodulating said first and second output signals of said first and second
multipliers.
7. An amplifier arrangement as claimed in claim 6 and including an AC
amplifier coupled between the first and second multipliers and the
demodulating means.
8. An amplifier arrangement as claimed in claim 6 and including a DC
amplifier coupled between the demodulating means and said correction
means.
Description
The present invention relates to amplifiers and in particular to high power
amplifiers wherein feed forward cancellation is employed, such as those
used in radio-frequency (r.f.) applications.
In amplifier design, there is a trade off to be made between distortion
performance and efficiency. Amplifiers which operate under so-called
`Class A` conditions have good distortion but low efficiency whereas an
amplifier operated under class C conditions is reasonably efficient but
introduces significant distortion. High efficiency and low distortion is
the goal, but efficiency increasingly becomes a consideration at high
power levels. For example, a typical cellular radio multi-carrier base
station amplifier requirement would be 200 w average, 2 kw peak so clearly
efficiency must be the best achievable if undue heat dissipation in the
amplifier is to be avoided. Unfortunately, the common technique of
negative feedback to correct distortion can only be considered for narrow
band applications in r.f. high power amplifiers. For this reason many r.f.
power amplifier operate in class A with the consequence heat dissipation
tolerated.
As an alternative to class A operation, a more efficient class AB amplifier
may be employed if feed-forward cancellation is applied. In this
technique, the amplifier output (suitably scaled) is compared with the
input signal to yield an error signal. The error signal is amplified and
reintroduced to the output 180.degree. out of phase with the original
distortion, the distortion products being thereby cancelled in the final
output. Feed forward can yield a 30 dB improvement in distortion
performance but only if both the first comparison loop and the second
correcting loop are accurately aligned. Generally the loop parameters do
not remain constant over the full operating range of the amplifier and
some dynamic correction is required, particularly in wideband application.
For example, the error signal may be subject to amplitude and phase
control prior to amplification. To provide the required control signals, a
pilot tone may be injected into the amplifier input. The pilot signal (at
residual distortion level) is detected at the output and used to provide
control signals.
There are a number of problems to be solved with this approach. Firstly,
there is successful detection of the low level pilot tone at the output.
This demands good filtering at the receiver. Secondly there is the problem
of deriving the control signals themselves. Systems which have been tried
are an amplitude only arrangement, with long time constants aimed at
correcting long term drift only, and iterative microprocessor controlled
phase and amplitude systems, the basis of the control algorithm being to
introduce an amplitude or phase change in the event of increased
distortion and monitor the effect at the output for improvement, then loop
until optimized. Unfortunately, this technique requires expensive
filtering techniques at the receiver and a processor resource making it
impractical and non-cost effective in most applications. Also, settling
time is likely to be long.
In accordance with the present invention there is provided an amplifier
arrangement to which feed forward cancellation is applied by a comparison
loop including comparison means for comparing amplifier input with
amplifier output to provide an error signal, a cancellation loop including
secondary amplifier means for amplifying the error signal and combining
means for combining said amplified error signal with said amplifier
output, a pilot generator coupled to said amplifier input to introduce a
pilot tone therein, detector means for detecting a level of pilot tone in
said amplifier output and correction means for correcting said
cancellation loop performance as a function of said detection wherein said
pilot generator is further coupled to a multiplier receiving said
amplifier output, said multiplier producing an output signal arranged to
control a loop parameter to effect said correction.
Preferably the loop parameter is amplitude, or phase, or both.
Advantageously, an amplifier arrangement in accordance with the present
invention includes a further multiplier, said further multiplier producing
an output signal arranged to control a further loop parameter. Preferably
there are provided means for controlling a parameter of the comparison
loop, for example phase.
In order that features and advantages of the present invention may be
further appreciated, embodiments will now be described, by way of example
only, with reference to the accompanying diagrammatic drawings, of which:
FIG. 1 represents an amplifier having feed forward cancellation in
accordance with the present invention; and
FIG. 2 represents an alternative embodiment of the present invention.
FIG. 3 represents a phasor diagram showing operation of the phase alignment
loop.
An r.f. high power amplifier 10 (FIG. 1) receives an input signal at its
input 11 via a coupling 12 whenever a signal for amplification is applied
to input port 14. The output of the amplifier 10 is tapped by a coupler 15
and fed to an input of a comparator 16, which may take the form of another
directional coupler. The input signal applied at port 14 is fed to a
second input of comparator 16 via a delay line 17, arranged to introduce a
delay substantially equal to that of the power amplifier 10. Comparator 16
thus produces at its output an error signal representative of the
difference between the input signal fed via amplified path a and delayed
path b. The amplified path a includes an attenuator pad 18 to compensate
for the signal gain introduced by the power amplifier 10. Paths a and b
constitute the comparison loop of a feed forward arrangement. The error
signal produced at the output of the comparator 16 is representative of
distortion introduced by the power amplifier 10 and is fed via networks
19,100 to an error amplifier 101 and thence to an input of a combiner 102,
which may take the form of a directional coupler. A second input of
combiner 102 receives the output signal of the power amplifier 10 via a
delay line 103 arranged to introduce a delay substantially equal to that
introduced by comparator 16, networks 19 and 100 and error amplifier 101.
Delay lines 17 and 103 may for example take the form of a length of
coaxial cable transmission line. Combiner 102 serves to introduce the
amplified error signal received via path c to the amplifier output signal
received via path d such that the error signal is in anti-phase with the
distortion present in the signal received via path d. The resultant signal
produced at output port 104 thus has had feed forward distortion
cancellation applied; paths c and d constituting the cancellation loop of
the feed forward arrangement.
An oscillator 105 is arranged to provide a pilot tone which is coupled into
the power amplifier input 11 by a coupling 106. It should be noted that
couplers 12 and 106 are arranged such that the oscillator signal is not
coupled into input signal path b. This may be achieved by providing
sufficient directivity from couplers 106 and 12. Since the pilot tone
travels via path a only to comparator 16, it will contribute to the error
signal and hence be cancelled by the cancellation loop. Thus the pilot
tone will contribute distortion to the eventual output produced at port
104 that is no greater than the residual distortion that remains
incorrected by the feed-forward loops.
The oscillator feeds a first input of a multiplier 107 via a coupler 108,
the second output of which is derived from the eventual output via a
coupler 109. As will be apparent to those skilled in the art when there is
amplitude mis-alignment the output of the multiplier provides a maximum
output when the residual pilot tone is in phase with the input signal from
the oscillator. In the general case where path 1 is not phase constant a
phase network 24 is controlled in such a way that the path behaves as a
constant phase path. When the loop is to be aligned, delay network 200 is
adjusted for maximum output from mixer 107. This output of mixer 107 is
fed as a control signal to network 19, being a controllable attenuator
used to adjust the amplitude of the error signal output of comparator 16
prior to amplification by error signal amplifier 101. Thus it will be
appreciated that amplitude errors in the cancellation loop resulting in
uncancelled distortion may be corrected by this mechanism.
Suppose, for example that the gain of amplifier 101 is less than its
nominal design value by -G. Correction network 19 must thus provide G less
attenuation to maintain alignment. In the non-aligned condition, some
portion of the pilot tone appears at output 104 and is thereby coupled to
an input of mixer 107. The mis-alignment produces a response of magnitude
E at the output of mixer 107 which is amplified by a control amplifier 201
to give a signal of magnitude G.sub.c.E driving network 19, where G.sub.c
is the gain of control amplifier 201. The sign of G.sub.c has to be such
that if E is produced by a lack of gain in amplifier 101, G.sub.c.E
reduces the attenuation in network 19. It is desirable that a high gain
amplifier 201 is provided.
In a way analogous to amplitude control network 19, network 100 is arranged
to control the phase of the error signal applied to error amplifier 101.
Network 100 is controlled by the output of multiplier 112 which receives
the eventual output signal via a coupler 110 at its first input and a
quadrature oscillator reference via 90.degree. coupler 111. It will be
appreciated that the output of multiplier 112 is representative of that
component of the pilot tone present in the eventual output at 104 that is
in quadrature with the oscillator signal; thus phase errors in the
cancellation loop resulting in uncancelled distortion are corrected.
For example, suppose there is amplitude alignment (phasors 31,32 of equal
magnitude in FIG. 3), but there is a phase unbalance. The remaining signal
at the output 104 due to the mis-alignment is represented by the phasor
33. Phasor 34 represents the quadrature component of the unbalance signal
which may be used as a basis for cancellation since for small .phi., /sin
.phi./ (phasor 34) is approximately equal to /.phi./ (phasor 33, to be
corrected).
Amplitude control network 19 may be a conventional PIN diode attenuator
with dc control and phase control network 100 may take the form at a
90.degree. coupler with a varicap diode controlled mismatched.
Advantages of the present invention will be apparent from the foregoing
description, in particular control of the cancellation loop for minimum
distortion allows better performance from the same power amplifier than
with prior art feed forward techniques or alternatively an amplifier with
a worse distortion performance (but better efficiency) may yield the same
overall performance. For a typical high power r.f. amplifier delivering
approximately 200 w average power (2 kw peak rated power amplifier, 200 w
peak rated error amplifier) operating in the 900 MHz band a reduction of
30 dB in distortion products could be achieved. The pilot tone may be
selected to be conveniently within the frequency range of interest,
control being wideband based on this representative performance.
Alternatively two or more pilot tones may be introduced at different
points across the band of interest, each controlling narrow band networks
to permit a yet improved cancellation loop control.
The performance of the arrangement described above may be improved by
incorporation of a further feature of the invention, and such an
embodiment will be described below. Continuing to consider the embodiment
of FIG. 1, it will be noted that the amplitude applied to the mixers is
preferably large, since the magnitude of the pilot tone is small (60 dB
down on amplified signal). If a signal of too large an amplitude is mixed,
however, inter-moduction occurs and the loop provides no correction. Hence
the mixer input signals must be relatively low level, yielding a small dc
output at the mixer for amplification by amplifier 201 to a level where
control may be effected. Therefore the loop is dependant upon the mixer dc
characteristics and it is generally the mixer dc stability that limits the
overall performance of the loop.
In further embodiment of the present invention (FIG. 2 in which reference
numerals common with those of FIG. 1 have been used for functionally
equivalent items) a low frequency oscillator 20 modulates the output of
the pilot tone oscillator 105 via a mixer 21. Modulated outputs of mixers
107, 112 respectively representative of the in phase and quadrature
components of the uncancelled pilot tone, are demodulated by envelope
demodulators 22,23 respectively to yield dc control signals to control
amplitude network 19 and phase network 100. It will be appreciated that
modulation of the pilot tone means that ac outputs are available provided
by multipliers 107 and 112 thus performance is not affected by multiplier
dc offset or drift.
Essentially mixers 107, 112 function as synchronous detectors, the envelope
output being amplified by ac coupled amplifiers 25, 26 respectively. Since
amplification is ac, performance is not dependent upon the dc stability of
the mixers. The envelope is detected by demodulators 22, 23, the dc
outputs of which may be amplified by amplifiers 201, 28. The gain of these
dc coupled amplifiers may be higher than the gain of the corresponding
amplifiers in the arrangement since any drift or offset of the mixers is
no longer coupled through. Hence the level of the pilot tone in the
overall may be reduced. In addition to their principle purpose of
amplification to overcome mixer dc offset and drift, the ac amplifier may
be arranged to contribute to the overall loop gain a proportion of the
control gain thus being provided by the ac amplifiers. The present
embodiment is particularly suitable for wideband applications. In such
applications it is important that the phase shift of the pilot tone on its
passage through the system is not so great that the phase margin of the
negative feedback arrangement is destroyed. In the present embodiment a
phase control network 24 is provided in the comparison loop for this
purpose.
In an example of the present embodiment for wideband operation a pilot tone
at 900 MHz was modulated by a low-frequency generator operating at 1 KHz.
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